Yamato and Musashi were the products of the IJN’s decisive battle doctrine. This doctrine was based on a hypothetical war plan with the USN. The idea was that the USN would need to sail across the vast Pacific to relieve an outpost (The Philippines for example). Along the way, the enemy fleet would be worn down by submarines and light forces, and then defeated in one single decisive battle, like Tsushima or Jutland. Remember, the Russian naval force in 1905 steamed all the way around the world only to be defeated in a single day. This historical battle had much influence on Japanese planners.

During the decisive battle the Japanese would unveil the true nature of the Yamato class battleships, which would be able to destroy their opponents using superior long range fire power from beyond the range that their opponents could overcome their superior armour protection, thus insuring victory in the decisive battle and winning the war in a single day. This meant that Yamato class battleships would need to be the biggest and most powerful ever built. Furthermore, the true nature of the designs would need to be kept secret.

There is some dichotomy to the design and to the doctrine. The design was based on the ideas of out ranging their opponent, and so based on the doctrine of long range gunnery, using aircraft spotting, popular during the 1920s. On the other hand, the IJN were masters of night battle which precludes generally long range gunnery. How these battleships were to be used in more realistic battle scenarios actually fought during WWII, 90% after 1941 actually fought at night, is a good question. Their fuel consumption was so great that Yamato and Musashi spent most of the war sitting in harbor, despite the capture of the Dutch East Indies oil fields early in the war. Late war, the two battleships were actually based right next to their oil supply and engaged in extensive night combat training exercises, using available Japanese radar technology.

Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.

The Yamato class design was based strictly on the strict all or nothing protection concept. Only the citadel, and turrets and barbets, were protected by armour, which was heavy. The main belts had a maximum thickness of 16-inches and were sloped 20*. This was capped by single armoured deck of 8-inch to 9 inches. At the bottom of the main belts was a lower belt to protect against underwater hits. When the previous Japanese battleship design, the Tosa, was cancelled it was used as a test target, and they found that many shells travelling an underwater trajectory had penetrated the hull. The joint between the main belt and the lower belt has received much criticism, as a weakness to the torpedo defense system. The torpedo defense system was unique in that it used no liquid loading. This was so that they would have flexibility in using a sophisticated pumping system to manage flooding, with counter flooding to counter lists and trims. Nonetheless there were massive areas fore and aft of the citadel that were highly vulnerable to both under water and shell/bomb damage.

The metallurgy used by the Japanese was copied directly from the British, who had been mentors to the IJN during previous decades. The construction steel was D-steel as used by the British. Post war chemical analysis indicates that the homogenous armour was exactly like British NCA. The face hardened armour was like that manufactured by Vickers. Yamato and Musashi used riveted construction. This meant that it was 15% heavier than it would be than if it used welded construction. That means it would have the same full load displacement as the Iowa class if it had been of welded construction.

Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.

Dave Saxton wrote: Yamato and Musashi used riveted construction. This meant that it was 15% heavier than it would be than if it used welded construction. That means it would have the same full load displacement as the Iowa class if it had been of welded construction.

IIRC, the weight difference between welding and riveting was about 2%.

With the Deutschland class the difference was calculated to about 15%, unless my sources are wrong. This is more plausible than 2%, considering that riveting requires overlap joints and much more framing.

Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.

Dave Saxton wrote:With the Deutschland class the difference was calculated to about 15%, unless my sources are wrong. This is more plausible than 2%, considering that riveting requires overlap joints and much more framing.

You have to a make a distinction from hull weight (IE the part of the ship that can be welded) and total weight at full load. Hull weight comes in at about ~30% of full load weight, so assuming that you could reduce hull weight by 15%, the overall weight reduction would be about 4.5%, but this would be a extreme case and ~1-2% is more likely with WW2 era welding technology.

Dave Saxton wrote:With the Deutschland class the difference was calculated to about 15%, unless my sources are wrong. This is more plausible than 2%, considering that riveting requires overlap joints and much more framing.

You have to a make a distinction from hull weight (IE the part of the ship that can be welded) and total weight at full load. Hull weight comes in at about ~30% of full load weight, so assuming that you could reduce hull weight by 15%, the overall weight reduction would be about 4.5%, but this would be a extreme case and ~1-2% is more likely with WW2 era welding technology.

WWII welding technology is not significantly different from today's welding technology. Nonetheless, it would depend on how much of the armour, machinery, and armament weights are also involved. Face hardened armour can not be welded but most homogeneous armour can be.

Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.

Dave Saxton wrote:With the Deutschland class the difference was calculated to about 15%, unless my sources are wrong. This is more plausible than 2%, considering that riveting requires overlap joints and much more framing.

You have to a make a distinction from hull weight (IE the part of the ship that can be welded) and total weight at full load. Hull weight comes in at about ~30% of full load weight, so assuming that you could reduce hull weight by 15%, the overall weight reduction would be about 4.5%, but this would be a extreme case and ~1-2% is more likely with WW2 era welding technology.

WWII welding technology is not significantly different from today's welding technology. Nonetheless, it would depend on how much of the armour, machinery, and armament weights are also involved. Face hardened armour can not be welded but most homogeneous armour can be.

HMS Ark Royal was designed for welding as far as was possible:

The ship was designed for the extensive adoption of welding. About 65% of the structure was welded (9) including the whole of the first 100ft from forward; the estimated approximate saving in weight as compared with riveting being 500 tons.
Brown, The Design and Construction of British Warships 1939-45 Major surface Vessels.

Hull weight was 13700 tons and deep displacement 27700 tons. The actual saving in weight was therefore about 3% of hull weight (14200 ->13700) and about 1.8% of full load. The maximum possible savings would have been about 770 tons for a ~5% saving in hull weight and ~2.7% in deep displacement.

dunmunro wrote:
HMS Ark Royal was designed for welding as far as was possible:

The ship was designed for the extensive adoption of welding. About 65% of the structure was welded (9) including the whole of the first 100ft from forward; the estimated approximate saving in weight as compared with riveting being 500 tons.
Brown, The Design and Construction of British Warships 1939-45 Major surface Vessels.

Hull weight was 13700 tons and deep displacement 27700 tons. The actual saving in weight was therefore about 3% of hull weight (14200 ->13700) and about 1.8% of full load. The maximum possible savings would have been about 770 tons for a ~5% saving in hull weight and ~2.7% in deep displacement.

The Ark Royal example is atypical. I have studied the British ship welding technology during WWII and they did not realize much of the weight savings possible. In many cases they retained much of the framing of riveted design and the overlap joints and replaced the rivets with fillet welds.

Perhaps the Japanese would have done so as well had they utilized welding since they were much influenced by British practice? However, there remains considerably more weight savings possible, by using the techniques used by American and particularly German shipyards.

Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.

Welding technology wasn't really secret. It had been developed in the private sector for the most part. The reason the Japanese eschewed the use of extensive welding in warships was because they had had bad experiences with it previously. The main reason for this bad experience was because they used Ducol construction steel for warship construction. Ducol exhibited poor weldability.

The Germans were certainly reluctant to share technology with their Axis allies. For example, when they sent a Seetakt radar to the Italians they omitted the really important bits that gave Seetakt its accuracy performance. Early in the war they also were reluctant to share technology with the Japanese. However, as the war progressed the Germans became more forthcoming with the Japanese.

It was the Japanese who didn't reciprocate. They ignored the radar scientist the Germans sent to help them and didn't really utilize the radar technology, such as Wuerzburg, that the Germans sent them. For their part the Japanese didn't share technology they had developed, such as their cavity magnetron, with the Germans.

Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.